When looking for a new home, it’s normal to draw up a list of must haves – good transport perhaps, and a garden. Compromises are not unusual, either. We may, for instance, forego a nice location for the sake of convenience.

The animal kingdom is no different. Symbion pandora sets up camp on the unappealing hairy mouthparts of Norwegian lobsters, in order to snack on the lobster’s leftover food. Some individuals will go to great lengths for all the mod cons, even if it means living in the most dangerous of neighbourhoods.

Camponotus schmitzi is one of them. This species of carpenter ant fearlessly sets up its nest inside the cavity of the carnivorous pitcher plant Nepenthes bicalcarata.

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Pitcher plants prey on insects that slip into their “pitfall trap” – a deep cavity filled with digestive fluid. So when it comes to insect real-estate, the inside of a pitcher should be a no-go zone.

On the flip side, the place does come with a pool – although for most ants this would be an unwelcome addition. Ants can paddle clumsily with their legs if they accidentally fall into a body of water, but wouldn’t go swimming through choice.

C. schmitzi – also known as the diving ant – is a clear exception. Endemic to the island of Borneo, it lives off arthropods and the nectar produced by pitcher plants. While other species of ant patrol their territories looking for food, diving ants stay discreetly tucked away behind the rim of the pitcher opening.

When it is time to feed, they sprint down the inner wall of the pitcher, dive head first into the digestive juices, swim to the bottom and fish out arthropods that have fallen into the trap.

The fact that they only dip briefly into the pitcher could explain how they survive&colon; they simply don’t stick around long enough to be digested.

Underwater sprinters

To find out how the ants manage to swim underwater, Holger Florian Bohn from the University of Freiburg in Germany and colleagues filmed them and created 3D models of their leg movement. They found that the ants moved their legs underwater in the same way as when running on land&colon; all six legs moved in a tripod sequence – three legs first then the other three.

Effectively, the ants were sprinting underwater. That suggests the behaviour is a relatively recent adaptation, unlike the complex swimming gaits seen in truly aquatic insects.

The fact that the ants can dive into the fluid is also puzzling&colon; they appear to be unaffected by surface tension, which would normally keep ants floating at the surface. Bohn suspects that the ancestors of C. schmitzi swam like other ants on the top of the pool, but small changes in the surface chemistry of their cuticle could have made it less water-repellent, eventually allowing them to break through the surface of the liquid and fully submerge themselves in order to fish for food.

The question then is what’s in it for the plant, which is being robbed of its food supply?

Last year, a French team led by Vincent Bonhomme, then at the University of Montpellier, found that the ants selectively fish out large arthropods from the pitcher fluid, then throw uneaten body parts back into the pool. The ants appear to play a crucial role in helping the plants digest their food.

Another theory is that the ants help clean the bottom of the pool by preventing a build-up of detritus, which could putrefy in the water. And everybody knows that a tenant that keeps the place clean is almost as hard to find as a decent place to live.